The present invention relates to the mounting of belt driven accessories, such as an alternator and an air conditioning compressor, and certain nondriven accessories, such as a coolant conditioner element, on heavy duty truck engines and, more particularly, to a modular accessory mounting apparatus incorporating various accessory mountings into a singular mounting and attachment structure which accommodates not only an alternator mounting with alternator drive belt adjustment capability and a coolant system conditioning element mounting, but also an air conditioning compressor mounting and an automatic drive belt tensioner therefor.